As a method for forming a nitride semiconductor layer on a sapphire substrate, a method disclosed in PTL 1 is known.
The method disclosed in PTL 1 is a method for growing a nitride semiconductor by a molecular beam epitaxy (MBE) method, and includes a step of forming a buffer layer on a substrate, a step of forming a first nitride crystal on the buffer layer at a first temperature, and a step of forming a first nitride crystal on the first nitride crystal at a second temperature higher than the first temperature.
The method disclosed in PTL 1 is intended to grow a nitride semiconductor with low crystal defect density on a sapphire substrate or the like by the MBE method having advantages, such as low residual impurity concentration.